The aim in the proposed research is to determine the atomic three-dimensional structure of a number of peptides that perform a variety of functions such as ion transport, analgesic, toxic, antitoxic and antibiotic by means of single crystal x-ray diffraction analysis. These crystals are composed of molecules containing light atoms only, C, N, O and H. The method of solution will be direct phase determination using the tangent formula and a variety of auxiliary formulas. Biologically active peptides often contain D-amino residues as well as L-amino residues and occur with cyclic backbones as well as linear backbones. Conformations are usually not predictable and are quite unique for different types of peptides. An immediate goal is to determine which factors govern conformation, e.g. amino acid sequence, chirality sequence, length of peptide, polarity of solvent molecules, bound water, ion complexation. The ultimate goal is to correlate structure with function, e.g. to understand the mechanism of ion capture and release by ionophores and ion transport by channel forming peptides. The specific peptide structures to be studied in this proposal are: a dodecapeptide from elastin; gramacidin A; two inactive analogs of the antitoxin antamanide; [Met5]enkephalin; a cyclic analog of enkephalin; a bicyclic nonapeptide; pteroyl hepta-gamma-glutamate and perhaps others.